That doesn't seem quite the same thing.  I assumed you meant that for some business reason some clients can't ever be nullable.  If it is simply a case of a client having systems will have an entry and others without systems will have a null in that column, then it's straightforward. 

In this case, however, I would consider an entry in the parent table that said no system and was represented by a blank string or another code.  Null is best used where the state is uncertain.  If it's not there and known to be not there, an entry according that would probably be better.